Mundane window conspiracy

I've worked in property maintenance for 10+ years. Every window company does design changed every 3 years or so. There must be extra parts in existence but everything i try to source something that's more than 2 or 3 years old it's impossible to find. I'm convinced they destroy their old stock. They got too good and without planned obsolescence they would go out of business. Excluding the low hanging fruit that is apple products(pun intended) and cell phones in general I'm curious what people in other industries have noticed.